1. Technical Field
The present invention relates to an illuminating device suitably used in electro-optic devices.
2. Related Art
Liquid crystal devices, which are a type of electro-optic device, have a backlight at the rear side of the liquid crystal display panel for transmissively displaying images. In general, the backlight is a illuminating device including a light source, such as an LED (light emitting diode), and an optical waveguide allowing the rear surface of the liquid crystal display panel to be irradiated two-dimensionally with light emitted from the light source. A main type of backlight has a light source at an end surface of the optical waveguide and is, thus, called “edge light type”.
In particular, vehicle-mounted backlights are larger than portable backlights and require high brightness. However, the LED used as the light source in known portable backlights has low luminous energy and inferior in reliability. Such an LED is not suitable as the light source of the vehicle-mounted backlight. Accordingly, the vehicle-mounted backlight uses as the light source a power LED (hereinafter LED referred to as power LED) that can use a high making current and consequently emit light with high brightness.
Unfortunately, as the making current is increased, the junction temperature of the LED is increased. Consequently the characteristics of the LED are varied and the reliability is degraded. In order to lower the junction temperature, the backlight requires a structure for releasing heat generated from the LED.
Illuminating devices or the like having a structure for releasing heat have been disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 2004-186004, 2005-283852 and 2002-162626 and Japanese Registered Utility Model No. 3098463. In the illuminating device disclosed in Japanese Unexamined Patent Application Publication No. 2004-186004, the LED light source is provided so that heat is conducted to the metal chassis. Consequently, the heat generated from the LED light source is released by heat conduction to the metal chassis, so that the increase of the LED light source temperature can be prevented. Thus, thermal breakage and breaking of wire can be prevented.
Japanese Registered Utility Model No. 3098463 has disclosed a heat radiator for a backlight-including LCD in winch a metal substrate connected to the LED is bonded to a metal housing case with a heat-conductive adhesive. In this structure, heat generated from the LED is absorbed by the metal substrate and immediately conducted to the metal housing. The metal housing case evenly releases the heat from the surface so that the LCD does not retain the heat.
Japanese Unexamined Patent Application Publication No. 2005-283852 has disclosed a liquid crystal display device. In this device, an LED chip is bonded to a mounting metal film of an LED-mounting substrate with an adhesive, and the mounting metal film and a metal film pattern formed on the chip mount surface are connected to a heat radiation metal film at the rear surface side of the substrate through a metal through hole. Consequently, heat generated at the chip mount surface of the LED-mounting substrate when the LED chip lights up is conducted to the heat radiation metal film from the mounting metal film and the metal film pattern through the metal through hole, and thus the increase of LED chip temperature can be prevented.
Japanese Unexamined Patent Application Publication No. 2002-162626 has disclosed a light source for liquid crystal display. In the heat radiator of this light source, a plurality of LEDs are mounted on a film substrate with a wiring pattern. The rear surface of the film substrate is bonded to a metal support frame with a heat-conductive adhesive. In addition, communicating holes are formed in the film substrate and the support frame corresponding to the LEDs so as to avoid lands, and the holes are filled with a highly heat-conductive adhesive filler. Thus, heat from the light source chip is conducted to the film substrate mainly through the heat-conductive adhesive filler and released. The heat conducted to the film substrate is further conducted to the support frame and released. Thus, the radiator can release the heat from the light source effectively.
However, the above-mentioned Japanese Unexamined Patent Application Publication No. 2004-186004 disclosing an illuminating device and Japanese Registered Utility Model No. 3098463 disclosing a heat radiator for backlight-including LCDs do not describe the heat radiation route from the heat source, or the LED light source, and they do not necessarily provide optimal heat radiation structures.
Although Japanese Unexamined Patent Application Publication No. 2005-283852 disclosing a liquid crystal display device and Japanese Unexamined Patent Application Publication No. 2002-162626 disclosing a heat radiator of a light source for liquid crystal display describe the heat radiation route from the LED light source, the insulation between the mounting substrate and the metal case or between the film substrate and the frame is not taken into account sufficiently and is not ensured accordingly.
In particular, the liquid crystal display device of Japanese Unexamined Patent Application Publication No. 2005-283852 has a complicated heat radiation structure in, for example, the mounting substrate and is accordingly difficult to mass-produce. In this liquid crystal display device, the mounting metal film and metal film pattern for heat radiation and the LED chip-driving metal wire are formed on one surface of the mounting substrate. Therefore, if the areas of the mounting metal film and the metal film pattern are increased from the viewpoint of heat radiation, it becomes difficult to route the driving metal wire and the conductivity is degraded. If the driving metal wire is routed while the areas of the mounting metal film and the metal film pattern are increased, the width of the mounting substrate is disadvantageously increased. In contrast, if the areas of the mounting metal film and the metal film pattern are reduced for ease of wire routing, the heat radiation performance is undesirably degraded.